Thermionic valve



\ April 23, 1940. c, s, BULL THERMIONIC VALVE Filed May 27, 1938 2Sheets-Sheet 1 Fkyw. I

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CONTROL ELtcTR INVENTOR CABOT SEATON BULL ATTORNEY April 23, 1940. c. s.BULL 2,198,330

THERMI ONIC VALVE Filed May 27, 1938 2 Sheets-Sheet 2 INVENTOR CA BOTSEATON BULL BY w ww ATTORNEY Patented Apr. 23, 1940 UNITED STATESTHERMIONIO VALVE Cabot Seaton Bull, Hillingdon, England, assignor t toElectric & Musical Industries Limited,

Hayes, Middlesex, Great Britain England, a company of Application May27, 1938, Serial No. 210,302 In Great Britain June 1, 1937 9 Claims.

This invention relates to thermionic valves of the type in which a spacecharge limited current is adapted to flow between cathode and. anode;The invention is therefore distinguished from certain other types ofvalves in which the current from the cathode is controlled not by spacecharge limitation but by deflection of the electron beam.

The chief object of the present invention is to provide an improvedthermionic valve in which the disadvantages of wound grids are avoidedand to provide a valve of simplified manufacture which may easily bemade to have a variable mu characteristic or to have a sharp cutoff sothat the slope of the grid voltage anode current characteristic changesonly slightly as the grid bias approaches the Value at which anodecurrent be- This disadvantage, it has been found, arises from the factthat when the control electrode is disposed close to the cathode thecontrol exercised on the cathode is not uniform around the surface ofthe cathode and, consequently, a large bias is required on the controlelectrode before the control electrode can exercise a sufficient controlaround the whole area of the cathode to reduce the anode current tozero.

According to the invention the control electrode is placed at asufiicient distance from the cathode for the control exercised on theelectrons on leaving the cathode to be substantially uniform around thearea of the cathode. Inone embodiment of the invention a thermionicvalve comprising at least a cathode, a control electrode and i an anodewherein the cathode is rectilinear or rod-like, has a control electrodealso like a rod in a common plane with the cathode and so positionedthat the minimum distance between the cathode and control electrode issubstantially constant along the length of said electrodes, thelnterelectrode spacing be ing such that the distance of the controlelectrode from the cathode is sufficiently large compared with the crosssection of the cathode and l the spacing of the anode or of any otherpositive relative values of the electrode potentials producing saidfield under normal operating conditions.

For the purpose of this specification normal operating conditions meansthe condition in which a space charge limited current flows and in whichthe range of anode currents and voltages is in the ratio of from 5 to10:1. For example, with a triode output valve the anode voltage may varybetween volts and 350 volts whilst the 10 anode current may vary from m/A. to 10 m/A.

The cathode either 'of the directly or indirectly heated typeispreferably linear and of a constant cross section along its length,the cross 15 section being either circular or rectangular. In the caseof a cathode of circular cross section the current leaving each unitarea of the cathode is substantially constant around the whole surfaceof the cathode evenwhenthe relative values of grid and anode voltagesarechanged, in such a manner that the same current leaves the oathode.Where a cathode is employed of a rectangular cross section the currentleaving each unit area of the cathode may not be uniform from point topoint but the distribution will be again independent 01' the relativevalues of control electrode and anbdepotentials causing the current toleave the cathode. The distance of the control electrode from thecathode should be at least two-and-a-half times the cathode diameterwhen the latter is of circular cross section and two-and-a-half timesthe longest dimension of the cathode cross section when the latter is ofrectangular form no matter which way the cath- 35 ode is orientated withrespect to the control electrode.

The invention may also be applied to tetrode, pentode and othermuti-electrode valves, the additional 'electrodesbeing of any suitableform 40 according to known "practice and arranged at suitable distancesfrom the cathode to perform their required functions.

The position of the anode, which is preferably cylindrical, or of thescreening electrode is rela- 45 tively unimportant providing that nopoint of the positive electrode (anode or screening electrode) is closerto the cathode than the distance of the control electrode from thecathode. For practical purposes the control electrode will seldom 50 beof smaller cross. section than the cathode. Preferably the control rodis a substantially larger cross section than the cathode and may toadvantage be of a diameter 5 to 10 times that of the cathode.

The invention is particularly suitable for valves which are designed forthe generation of oscillations of very high frequencies, such as thoseencountered. in television systems, and higher;

since the capacity between the cathode and the control electrode can bemade very small and the time of flight ofthe electrons very short.

By employing a valve in accordance with the invention which can bereadily constructed with a low amplificationfactor larger currents canbedrawn by the anode and because of the larger current fiowing the time offlight of the electrons is reduced so that very high frequencies can beobtained.

If desired, the control electrode may comprise a plurality of rods andtwo or more cathodes may be provided, each cathode being betweenadjacent control rods, the cathodes and the rods of the controlelectrode being in the same plane. Alternatively, the cathodes and therods of the control electrode may be on, the circumference of animaginary cylinder.

Since in the preferred form of the invention both cathode and controlelectrode are in the form of straight rodsthey can be manufactured withvery little variation of their dimensions throughout their length,enabling them to be easily mounted in the same plane and parallel to oneanother. The fact that the control electrode may consist of a thickWire, and is not of the usual wound construction, enables it to bedisposed sufficiently close to the filament to permit of a high mutualconductance to be obtained without increased microphonic or mechanicaldifficulties and enabling the time of the flight of the electrons to bereduced and topermit of the production of valves having lowamplification factors which are eminently suitable both for thedetection and generation of oscillations of very high frequencies. I

In order thatthe invention may be clearly understood and readily carriedinto effect the same will now be more fully described with reference tothe accompanying drawings in which:

Figure 1 is a diagrammatic cross sectional view of a triode valveconstructed in accordance with the invention,

Figure 2 is a view similar to Figure 1 but illustrating a modification,

Figure 3 illustrates a modification of the invention in which thecontrol electrode comprises a pair of rods,

Figure 4 illustrates a modification in which the control electrodecomprises a plurality of rods and a plurality of cathodes. alsoprovided.

Figure 5 illustrates a further form of the invention,

Figure 6 illustrates yet another form of the invention as applied toatetrode valve,

Figure '7 illustrates two characteristic curves of the valve shown inFigure 6, and

Figure 8 of the drawings illustrates one form of variable-mu valveconstructed in accordance with the invention and Figures 9, 10, 11 and12 illustrate further forms of variable-mu valves.

As shown in Figure 1, the valve comprises a cathode H), a controlelectrode II and a cylindrical anode I2. The cathode I0 is circular incross section and is a straight rod. The cathode may be indirectlyheated or directly heated and mounted in the conventional way. Thecontrol electrode H comprises a straight rod of circular cross sectionarranged parallel to the cathode ID, the spacing of the cathode andcontrol electrode being at least two-and-a-half times the diameter ofthe cathode Hi. The electrodes of the valve will of course be mounted ina highly evacuated envelope. cylinder but optionally of other shapes,such as a fiat plate, is disposed at a distance from the oathode, notless than the distance between the oathode lll and the control electrodeI I. p

In Figure 2 the cathode ID is of the indirectly heated type andcomprises an outer casing of rectangular form and coated with a suitableelectron-emitting substance, and is provided internally with a heater,not shown. The control electrode H comprises a rod. mounted parallel tothe cathode Ill and the distance between the cathode and the controlelectrode is at least twoand-a-half times the longest dimen sion of thecathode cross section.

The construction of valves shown in Figures 1 and 2 are eminentlysuitable where low amplification factors are required, but where highamplification factors are necessary the control electrode may comprisetwo or more rods.

Figure 3 of the drawings illustrates such an arrangement in which twocontrol rods H are mounted on each side of the cathode Ill. In thisarrangement the exact centering of the cathode in the plane containingthe axes of the rods H is not critical but care should be taken toensure that slight deviation of the cathode occurs in a direction atright angles to the said plane.

Figure 4 illustrates a somewhat similar construction in which thecontrol electrode comprises four rods H and a plurality of cathodes itis provided, a cathode It] being disposed between adjacent rods H, asshown.

Figure 5 illustrates a modification of the arrangement shown in Figure4, in which the control electrode comprises six rods ll arranged on thecircumference of an imaginary cylinder indicated by the dotted line 13and six cathodes.

H! are provided each cathode being disposed between adjacent rods II andalso arranged on th circumference of the imaginary cylinder I3. In thisarrangement an additional anode 12a is provided arranged concentricallywith the imaginarycylinder I 3. In some cases the anode l2 may beomitted and the inner anode lZa alone employed. With constructionsemploying up to three rods constituting the control electrode, it ispreferable to employ cylindrical anodes and Where additional electrodesare employed these should also be of cylindrical form since somereduction in the control electrode input capacity can be thus obtained.

In a valve manufactured in accordance with the invention and employingtwo filaments, each 1 mm. apart and 20 mms. long and of a diameter equalto 0.05 mm. together with a control electrode comprising three rod-likeelements of 0.25 mm. diameter and 1 mm. apart, together with an anode inthe form of a cylinder 4 mms. in diameter, the control electrode/anodecapacity was found to be 1 to 2 cms. the amplification factor 3 and themutual conductance 2.2 m/A. per volt. In a comparable valve having afilament 40 mms. long and 0.03 mm. diameter bent into a V-shape with awound elliptical grid with a minor axis of 1 mm. and supported on tworods each 1 mm.

in diameter and wound with molybdenum wire of The anode 12 shown as afactor 15 Such a valve could not be manufactured with an amplificationfactor as low as 3 without considerable difiioultyr Although it ispossible to employ screening electrodes in a valve in accordance withthe invention, which screening electrodes may comprise wire woundhelices or meshes or a single rod it is preferable to employ as thescreening electrode one consisting of rods similar to and parallel tothe rods constituting the control electrode and aligned therewith, sincein this case, so-c alled screened grid and other types of valves can beconstructed having a screen current as low as or less of the cathodecurrent. I A valve constructed in this manner isshown in Figure 6 withtwo cathodes l and three rods ll constituting the control electrode. Oneach side of the control electrode three rods M are provided arranged inplanes parallel to the plane containing the control electrodes andcathodes, the rods l4 being parallel to one another and aligned with therods ll. Additional screening plates 55 are provided, preferablyconnected directly to the cathode, carried by support bars Hi, theelectrodes so far described being surrounded by a cylindrical anode l2supported by support bars it. The

cathodes it may be 0.05 mm. diameter and the I rods i may be 0.5 mm.- indiameter and the distance between the centres of adjacent rods H may be1 mm. The rods l4 may be 0.25 mm, in diameter, and the centres of therods 14 may be spaced from the centres of the rods H a distance of 1 mm.The anode I2 may have a diameter of 15 mrns. whilst the width of theplates 55 may be 7 norms. and spaced from the centre of the outermostrods II a distance of 1 mm. Two characteristiccurves for a valveconstructed in accordance with Figure 6 and with the climensions givenare shown'in Figure '7. From the first curve it it will be observed thatwith a voltage of 80 volts applied to the screening electrode and withan anode voltage of 150 volts the anode I current is reduced to zero atabout 11 volts negative potential on the control electrode. The secondcurve l0 indicates the condition in which a voltage of 50 volts isapplied to the screening electrodeland a voltage of 150 voltsis appliedto the anode, in this case, the anode current is reduced to zero atanegative potential of 7 volts. The ratio of the voltages for out off,i. e., 11:7, is substantially equal to the ratio of the screen voltages,namely 80:50. The valve has therefore practically no tail, 3

It is found that the electron stream leaving the cathode can be made totake the form, in the case where at least two rod like elements'arew thecathode and by choosing appropriate operating potentials. Thearrangement shown in Fig. 6 can produce the eifectmentioned. Thescreening electrode may take the form of a slotted plate or cylinder orcomposed of rods, the electron stream passing through the slit 01' slotsor the space or spaces between the rods. In this way the input capacitycan be reduced.

The control electrode may consist of a rod or rods which aresufficiently thick to enable heat to be conducted along them readily.Emission from the control electrode and leakage can therefore be reducedby joining the rods of the control electrode to one or more radiatingfins at their ends. Alternatively, in very large valves, the said rod orrods may consist of tubes, means being provided for passing coolingliquid therethrough, or may be terminated at one or both ends in metalblocks which are cooled by liqihd. These features can be appliedto anytype of valve using rods as the control electrode.

Whilst, as stated above, the invention has primarily been developed witha View to providing a valve having very little tail, nevertheless, thegeneral construction of valve described in Figures 1 to 6 can be alteredso as to provide valves with a predetermined degree of ,tail to functionas, variable-mu valves. Figures 8 to 12 show various forms of thisaspect of the invention and in all of these figures the cathode isindicated by the reference numeral H], the control electrode by thereference numeral I I and a cylindrical anode, which is shown inlongitudinal section, by the reference humeral [2. In these figures thecontrol electrode need not be displaced from the cathode a distance atleast equal to two-anda-half times the cathode diameter, although if thecontrol electrode is so disposed, some advantages accrue.

In Figure 8 the control electrode H is shown of a nonuniform crosssection ahd isin the form of a cone. It will be appreciated that withthis construction the control electrode will exercise different controlover different portions of the electron stream which is the essentialfunction i of a variable-mu valve.

In Figure 9 the control electrode H is bent along its length, such anarrangementalso affordingra variable-mu characteristic, the amount ofbending of the control electrode H determining the amount of tail.

Figure 10 illustrates a further form in which] the control electrode His in the form of a rod of uniform cross-section and the cathode I0 isof parallel. This c'nstru ction also affords a variable-mucharacteristic.

In the examples described in Figures 8 to 12 thecathodes shown inFigures 8, 9, l1 and 12 may be either of the directly or indirectlyheated type, but the cathode shown inFigure 10 is preferably of theindirectly heated type since it would be difficult to manufacture adirectly heated cathode of the form shown in Figure 10. The cathodes Illand the control electrodes I! are preferably of circular form in'crosssection, but of course other sections may, if desired, be employed.Although in each of the examples described in these figures a single rodis employed as the control electrode, more than one rod and more thanone cathode may, if desired, be used. Additional electrodes may beassociated with the valves, such as screening electrodes, in knownmanner.

I claim:

1. An electron discharge device comprising a rectilinear cathode ofuniform cross-section throughout its length, an anode electrode having asurface parallel to the surface of said cathode,

and a rectilinear control rod of greater diamsaid anode electrode beingspaced from said T if) cathode to make-the electron accelerating fieldat said cathode substantially uniform about said cathode. p p

2. An electron discharge device as defined in claim 1 in which thecathode is a filament and the control electrode is a straight rod of adiameter several times that of the cathode.

3. An electron discharge device as defined in,

I claim 1 in which the cathode is a straight filament and the controlelectrode is a straight rod having a diameter about'ten times that ofsaid cathode and spaced iromsaid cathode a dispositioned between and inthe plane of the longitudinal axes of said rods and spaced from each ofsaid rods a distance several times the diameter of said cathode, and anaccelerating electrode comprising a pair of rectilinear parallelrods-interposed between said anode and said control electrode with eachrodoi said control electrode coplanar with the corresponding rod ofa'aid accelerating electrode in a plane transverse to the plane of saidcontrol electrode.

5. An electron discharge device including a anode, a control electrodecomprising a plurality of spaced rectilinear parallel rods, afilamentary cathode comprising rectilinear portions in the plane of thelongitudinal axes oi said rods, each of said portions being of smallerdiameter than the maximum diameter of said rods and positioned between apair of said rods and spaced from each of said rods a distance severaltimes the diameter of said cathode portion, and an acceleratingelectrode comprising a plurality of rectilinear parallel rodscorresponding to the rods of said control electrode interposed betweensaid anode and saidcontrol electrode with each rod of said controlelectrode parallel to and aligned with a corresponding rod of saidaccelerating electrode" in a plane which includes said correspondingrods and intersects said anode.

6. An electron discharge device including an anode, a control electrodecomp-rising two spaced rectilinear parallel rods, a rectilinearfilamentary cathode positioned between and in the plane of thelongitudinal axes of said rods and spaced from each of said rods adistance several times the diameter of said cathode, and an acceleratingelectrode between said anode and said control electrode and comprisingtwo rectilinear rods parallel to said control rods each of said rods ofsaid accelerating electrode being coplanar with 1 a corresponding rod ofsaid: control electrode in :a plane normal to the plane of said controlrods, said anode being imperforate and further from said cathode thansaid accelerating electrode.

'7. An electron discharge device as defined in claim 6 having a pair ofsheet electrodes at opposite ends of said control electrode andextending normal to the plane of said control electrode and connected tosaidcathode.

8. An electron discharge device including an anode, a pair ofrectilinearcontrol rods parallel to the surface of said anode, arectilinear cathode between and coplanar with said rods and spaced fromeach control rod at least two and one-half times the diameter of saidfilamentary cathode, said cathode having a diameter less than themaximum diameter of either control rod, and an accelerating electrodebetween said anode and said control rods comprising a pair ofrectilinear rods parallel to said control rods, each of saidaccelerating electrode rods being coplanar with a corresponding controlgrid red in a plane which intersects the surface of said anode.

9. An electron discharge'device comprising a tubular anode having apair, of diametrically opposite anode support rods, a pair of sheetscreening electrodes inside said anode and normal to the planeof saidanode support rods, a control electrode between said screeningelectrodes and comprising a plurality of rectilinear parallel controlrods, a cathode comprising a filament smaller thansaid control rods andhaving a plurality of rectilinear sections less in numher by one thansaid control rods, each section being between and coplanar with a pairof said control rods and spaced from said rods at least two and one-halftimes the diameters of said section, and an accelerating electrodecomprising rectilinear rods equal in number to said control electroderods, each of said control electrode rods being parallel to and coplanarwith a corresponding control rod in a plane normal to the plane of saidcontrol electrode.

CABOT SEATON BULL.

